In the art of powder metallurgy, there is a widely known process in which a metal or alloy powder is first compacted and then the green compact is heated at a relatively low temperature in a reducing or neutral gaseous atmosphere so as to obtain a sintered compact. According to this method, it is possible to obtain a sintered compact of a material which is usually hard to melt. Furthermore, the inclusion of impurities is minimized as compared with products obtained by more usual melting methods, and hence it is possible to obtain a product having excellent physical and other properties which cannot be expected from the ordinary products formed from molten metal.
This method, however, had the problem that, the sintered compact obtained by sintering a green powder compact is porous and also, in the case of a metal having high vapor pressure such as zinc, the component elements would volatilize away under the sintering heat, so that it would sometimes prove impossible to obtain a sintered blank of a desired composition. There are also materials that cannot be sintered by using this method. Thus, there has been a certain limitation as to the scope of products obtainable and the scope of use of these products. Such deficiencies have been particularly noticeable in the case of the sintering of high brass.
Therefore, the method generally employed for obtaining high-density metal-made machine parts was one of cutting solid materials or casting molten materials to the approximate size of the object product and then machining them into the desired shape. According to this conventional method, as adapted for instance to the forming of pressure-proof brass pipe parts as commonly used in industry, first the brass cutting scraps and molten material are fed into a melting furnace, followed by the addition thereto of zinc, lead and electrolytic copper to prepare a molten bath of the desired brass composition for forging. This molten bath is then shaped into an ingot, the latter being then rolled into an elongated brass bar of a desired size, and this bar is cut into a desired size to form billets. Then the cut billets are heated to around 700.degree. C., subjected to hot forging in a mold of a predetermined configuration and further passed through the steps of trimming, surface cleaning and machining to produce a desired pressure-proof pipe part.
According to such a conventional method, since the shapes of the billets which can be forged are limited, many sections are left that require machining after forging, resulting in increased working time and loss of material. Also, when the cutting scraps formed in the working are reduced in the melting furnace, when high brass is being manufactured, it is necessary to replenish the zinc component as zinc may evaporate during the process. This increases the cost of the heat source. In many of the currently used methods, the loss in weight of the material suffered during the machining of the forged blanks exceeds 50% in the case of pipe parts, as such parts are mostly hollow in shape.
In order to overcome these disadvantages, some forging machines have been developed which are specifically designed for the forging of hollow parts, but such machines have complicated die mechanisms, and are also expensive. Consequently, they have not yet found general acceptance.